Error: "Object reference not set to an instance of an object" found in Application Log
This error occurs when the Archive Manager Exchange Store Manager service account (by default called ArchiveMgr_ESM) or the "ArchiveManager Exchange Admins" group (as specified in the Install Guide) do not have full mailbox access permissions on the Private Store Containing the mailboxes. Typically this usually only affects one mail store on a server, and the others are fine. Also, upper level objects (eg the Storage Group, Servers, Administrative Group etc) will have the correct permissions.
It can very often occur when the above mentioned service account is made a member of the Domain Admins group which by default has Deny permissions on Full Mailbox Access.